The present invention relates to a rubber composition having low gas permeability. More particularly, the present invention relates to a clay filled butyl rubber composition having low gas permeability. Furthermore, the present invention relates to a process for the exfoliation of clay materials.
Butyl rubber is a synthetic elastomer typically comprised of a copolymer of isobutylene and isoprene. Butyl rubber was first produced with the advent of World War II and the associated limited supply of natural rubber.
Butyl rubber has a low degree of permeability to gases due to uniformity in the polyisobutylene portion of the butyl chains and the ease of packing provided by this uniformity. Butyl rubber can be 8-10 times more resistant to gas permeability than natural rubber and also has excellent resistance to heat, steam and water. The low degree of permeability to gases accounts for a frequent use of butyl rubber in inner tubes and tire inner liners. Similarly, butyl rubber is advantageously used in air cushions, pneumatic springs, air bellows, accumulator bags, and pharmaceutical closures. Furthermore, the thermal stability of butyl rubber makes it suitable for construction of tire-curing bladders, high temperature service hoses, and conveyor belts for handling hot-materials. Butyl rubber has also been used in damping mounts for engines and similar apparatus.
Notwithstanding its many desirable characteristics, in-chain unsaturation in butyl rubber, contributed by the presence of isoprene monomer units in the backbone, can be attacked by atmospheric ozone. These attacks may, over time, lead to oxidative degradation, which may subsequently lead to chain cleavage. This potential breakdown of the rubber could result in lower damping properties. One way to limit the impact of atmospheric gases on the butyl rubber structure is to further lower the gas permeability of the rubber composition.
It is known that the addition of exfoliated clays to certain types of rubber compositions may be an effective way to lower gas permeability. To lower gas permeability in rubber, the added clay must be of a small size, a condition traditionally achieved by exfoliation. Typical clays, prior to exfoliation, have a layered structure with a gap of about 0.1 nm between each layer and positively charged ions on the surface of each layer. The positively charged ions are attached by an ionic interaction with the negative surface of the clay layers, and create a net neutral charge between clay layers.
Traditional exfoliation is generally conducted as follows. Clay is first swelled by placing it in water. Swelling takes place because the positively charged ions become solubilized in the water, leaving adjacent negatively charged clay layers. The adjacent clay layers are repulsed by their similar negative charges, resulting in gaps of up to about 3 nm between the layers. An organic salt, such as cetyltrimethylammonium bromide or benzalkonium chloride is then added to the swollen clay to form an organo-clay. The organic salt is attracted to the negatively charged surface of the clay, keeping the swelling state stable and gaps of about 5-10 nm between the layers. This organo-clay is then dried and subsequently placed in an organic solvent, such as toluene. A polymer such as polypropylene or nylon is then added to further separate the layers of the clay. Moreover, the polymer is attracted to the organic salt and therefore penetrates between clay layers. The large molecule size of the polymer serves to counteract any remaining Van der Waals interactions between the layers and the clay becomes fully exfoliated, i.e. separated into discrete layers.
Previous attempts to incorporate exfoliated clay in butyl rubber have proven largely unsuccessful due to the poor interaction of butyl rubber with the organic salts used to make the organo-clay. Therefore, it would be desirable to develop an improved method of dispersing an exfoliated clay in butyl rubber to lower gas permeability.